Writing a short story may be a lot of fun, but it may also be the hardest and most frustrating of tasks because it is limited. It is a blessing that it is, for a short story gives you a sense of how long your writing will be. When writing a novel, you have much more room with which to work. Writing a short story requires that you use every word to its best advantage. You must maintain your focus, and word choice must be concise and right on the money. One of Mark Twain's best quotes about writing, states:

The difference between the almost right word & the right word is really a large matter--it's the difference between the lightning bug and the lightning.

In other words, don't settle for a word that almost says what you want: hold out for the perfect one.

Structure speaks to the essence of the entire short story. So first, I would plan to follow Fraytag's "dramatic arc," which we often call the plot diagram. It may seem elementary, but structure is absolutely necessary. In the late 19th Century, Gustav Freytag noticed a pattern in the five-act plays, which were around long before Shakespeare and his contemporaries started writing their dramas. However, the elements of "the arc" work in all kinds of literature:

...it can be applied (sometimes in a modified manner) to short stories and novels as well.

These elements include the exposition (introduction); rising action; climax; falling action; and, resolution (denouement), also known as revelation/catastrophe.

First piece of advice, write about what you know. Then, do a lot of research. Make every aspect of the story believable. And although you ask specifically about the introduction and conclusion, establishing the structure is important before you begin, and the "dramatic arc" does this. Many writers believe that one must almost start at the middle of the story before writing the introduction or conclusion. They choose the setting and then develop their characters, and know both intimately—especially the characters' story—before beginning with the introduction. This makes sense: we know someone so much better after we have established a relationship. Can we truly describe someone after knowing him only a week, or much more fully after knowing him a year? More depth to your characters makes them more engaging and more believable.

There is a great deal of information out there about how to write, and often the advice deals with essays. However, the fundamentals are the same. In writing an introduction for an essay or short story, be exact and concise, for this part of your writing...

...can tell readers how well your thoughts are put together, how well organized [you are]...

Engage your reader from the very start. Make them want to read your story. Make it humorous, exciting or interesting and engaging. Make sure each work counts. Don't ramble.

In terms of the conclusion, this is also based on what you know of your characters and their story. This is where you answer questions, explain mysteries, answer questions and tie off loose ends. This is critical. Here, you can finalize your "message" to the reader.

Truly, there is no set format for beginning and ending a short story. One can begin in media res or with a flashback; with the words of a character or first-person narrator. According to "The Crafty Writer,"

Your first sentence is crucial; it should be filled with energy, intrigue and forward momentum. The readers should be stopped in their tracks and not be able to turn away until they’ve read the whole thing.

What may be the most important component to the creation of the short story is that outlined by Edgar Allan Poe in his essay "The Philosophy of Composition": A short story must be brief and have a singleness of effect. This unity of effect and a logical method such as that of Freytag are essential.

According to Poe, the unity of effect is only achieved when the writer first decides how his/her story will end. Then, after having decided upon the ending, the writer should arrive at his/her usage of other pertinent matters such as tone, theme, setting, characters, and conflicts, and the direction of the plot. The plot will have "an indispensable air of consequence" if the author makes all these elements contribute to the development of the "overall intention" of the narrative. Then, the ending can be a sort of "clincher," another sentence that grabs in its final observation.